A martensitic stainless steel pipe has conventionally been used widely for such applications as oil wells because of its excellent resistance to corrosion caused by CO2. On the other hand, if all cooling operations for quenching in a heat treatment process are performed by water cooling, the martensitic stainless steel pipe is susceptible to quenching cracks because the material therefor has very excellent hardenability. Therefore, to quench the martensitic stainless steel pipe in the heat treatment process, a natural cooling method or an air cooling method in which air is sprayed toward the outer surface of steel pipe is generally adopted. The cooling method, however, requires much time to cool the pipe, and therefore the heat treatment efficiency lowers.
To eliminate the above disadvantage of low heat treatment efficiency as one purpose, for example, a method described in WO 2005/035815 (hereinafter, referred to as Patent Document 1) has been proposed. In the method described in Patent Document 1, a water cooling operation having a high cooling rate and an air cooling operation are combined by utilizing the fact that cracks are less likely to develop even if water cooling is performed in the temperature range excluding the vicinity of Ms point (a temperature at which the martensitic transformation of steel begins when cooling is performed at the quenching time). Specifically, Patent Document 1 discloses a quenching method in which after being heated to be austenitized, a steel pipe is cooled in the order of water cooling, air cooling, and water cooling.
Regarding the above-described air cooling operation, Patent Document 1 discloses an air cooling apparatus having a configuration such that the whole outer surface of steel pipe is cooled from the downside by a fan or a blower, and the inner surface of pipe end portion can be cooled by an air nozzle (paragraph 0062 of Patent Document 1).